1 LIMITATIONS OF DRIFT-DIFFUSION MODEL FOR STUDYING SPIN TRANSPORT IN NANOWIRES Sandipan Pramanik 1 , Supriyo Bandyopadhyay 1 , and Marc Cahay 2 1 Department of Electrical and Computer Engineering 2 Department of Electrical and Computer Virginia Commonwealth University Engineering and Computer Science Richmond, VA23284 University of Cincinnatti Cincinnatti, OH45221, USA ABSTRACT We study spin relaxation of upstream electrons in a multi subband quantum wire due to D’yakonov-Perel’ mechanism. Our study reveals that upstream spin relaxation is non-monotonic in space and has a complex dependence on electron distributions over various subbands. Classical drift-diffusion model of spin transport fails to account for these features and hence this is inappropriate for studying spin transport in quantum wires. INTRODUCTION Classical drift-diffusion model is widely used for modeling spin polarized transport in various systems [1]. However it has certain limitations. For example it fails to capture transport nonlinearities (like velocity saturation) which significantly influences spin relaxation [2]. Here we point out another shortcoming of this model. According to drift-diffusion theory, spin relaxation of upstream electrons is monotonic in space. In this work we show that this is certainly not true for quantum confined systems in which subband structure strongly influences spin relaxation. THEORY We consider a GaAs quantum wire with rectangular cross section of 30 nm x 4 nm as shown in Figure 1. A metal gate is placed on the top which causes a Rashba interaction in the wire but does not perturb the energy levels in the wire significantly. We assume that this gate electric field has a magnitude cm kV E y / 100 = . In addition, there is an electric field ( x E x ˆ − ; 0 > x E ) in the channel that drives electron transport. This configuration mimics the spin field effect transistor (spinFET) [3].